With reference to FIG. 10, a description is provided of a conventional discharge lamp operating system for a multi-lamp. In cold-cathode tube discharge lamp operating system 1, power supply 2 allows a plurality of discharge lamps 3 for radiating a panel to light up at the same time.
In recent years, with increases in screen size, it is required in a liquid crystal display that multiple discharge lamps 3 for radiating the panel be lit and the voltage be increased to support the high brightness of respective discharge lamps 3. In a discharge lamp operating system using multiple discharge lamps 3, the brightness of respective discharge lamps 3 need be equalized. When this balance is inaccurate, high brightness portions and low brightness portions appear on one liquid crystal display as brightness unevenness, which may make the screen difficult to see.
The variation in impedance between respective discharge lamps 3 is one of the causes for occurrence of such brightness unevenness. In order to prevent such brightness unevenness, balance coil 4 is incorporated into the discharge lamp operating circuit for a so-called multi-lamp system using a centralized power supply. Balance coil 4 is used to accommodate the impedance variations and to keep the balance of power to be supplied to respective discharge lamps 3. The balance coil suppresses brightness unevenness between discharge lamps 3. Such a discharge lamp operating system is disclosed in Patent Document 1, for example.
However, when balance coils 4 are used, N−1 pieces of balance coils 4 are necessary for N pieces of discharge lamps 3 in one discharge lamp operating circuit. An increase in the number of balance coils 4 increases the transmission loss at a power conversion, the number of components, the mounting area, or the production cost resulting from the increase in the number of components.    Patent Document 1: Japanese Patent Unexamined Publication No. 2004-335443